1. Field of Invention
The present invention relates to a plaster composition and a method of making the same. More particularly, the present invention relates to a plaster composition comprising a high molecular weight dispersant.
2. Description of Related Art
Gypsum is a mineral that predominantly comprises calcium sulfate dihydrate (CaSO4.2H2O). The partial dehydration of gypsum, typically via calcination, forms calcium sulfate hemihydrate (CaSO4.½H2O), which is generally known in the art as plaster. The calcination of gypsum at atmospheric pressure tends to form the beta-gypsum form of plaster whereas calcination of gypsum under elevated pressure tends to form the alpha-gypsum form of plaster. The dry plaster material can be mixed with water to form a variety of hardenable compositions such as, for example, molding plasters, skim coatings and joint compounds.
The setting reaction involves the reaction of calcium sulfate hemihydrate and water to form calcium sulfate dihydrate. The theoretical water required to convert the calcium sulfate hemihydrate to calcium sulfate dihydrate is only 18.7% by weight on a theoretical weight basis. However, considerable excess water is required to fluidize the dry plaster and to obtain a flowable slurry. The amount of excess water depends upon factors such as the form of plaster used and the presence or absence of additives in the plaster composition. For example, Lange et al., U.S. Pat. No. 4,184,887, disclose that naphthalene sulfonic acid formaldehyde condensation polymers having a molecular weight from about 300 to about 3,000 can be used as non-discoloring water reducers in plaster compositions.